1. PPT#3 Muscle Metabolism

2. Muscle Metabolism anaerobic fermentation aerobic respiration
all muscle contraction depends on ATP
ATP supply depends on availability of:
oxygen
organic energy sources such as glucose and fatty acids
two main pathways of ATP synthesis
anaerobic fermentation
enables cells to produce ATP in the absence of oxygen
yields little ATP(2) and toxic lactic acid, a major factor in muscle fatigue
aerobic respiration
produces far more ATP
less toxic end products (CO2 and water)
requires a continual supply of oxygen

3. Modes of ATP Synthesis During Exercise
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10 seconds
40 seconds
Repayment of
oxygen debt
Duration of exercise
Mode of ATP synthesis
Aerobic respiration
using oxygen from
myoglobin
Phosphagen
system
Glycogen–
lactic acid
system
(anaerobic
fermentation)
Aerobic
respiration
supported by
cardiopulmonary
function
Figure 11.18

4. Immediate Energy Needs
Example: short, intense exercise (100 m dash)
oxygen need is briefly supplied by myoglobin – rapidly depleted
muscles meet most of ATP demand by borrowing phosphate groups (Pi)
Phosphagene system: two enzyme systems control these phosphate transfers
myokinase – transfers Pi from ADP to ATP
creatine kinase – obtains Pi from a phosphate-storage molecule creatine phosphate (CP)
provides nearly all energy used for short bursts of intense activity
one minute of brisk walking
6 seconds of sprinting or fast swimming
important in activities requiring brief but maximum effort
football, baseball, and weight lifting

5. Immediate Energy Needs
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ADP
ADP Pi
Myokinase
AMP
ATP
Creatine
phosphate
ADP Pi
Figure 11.19
Creatine
kinase
Creatine
ATP

6. Short-Term Energy Needs
as the phosphagen system is exhausted
muscles shift to anaerobic fermentation
muscles obtain glucose from blood and their own stored glycogen
in the absence of oxygen, glycolysis can generate a net gain of 2 ATP for every glucose molecule consumed
converts glucose to lactic acid
glycogen-lactic acid system – the pathway from glycogen to lactic acid
produces enough ATP for 30 – 40 seconds of maximum activity

7. Long-Term Energy Needs
after 40 seconds or so, the respiratory and cardiovascular systems “catch up” and deliver oxygen to the muscles fast enough for aerobic respiration to meet most of the ATP demands
aerobic respiration produces 36 ATP per glucose
efficient means of meeting the ATP demands of prolonged exercise
one’s rate of oxygen consumption rises for 3 to 4 minutes and levels off to a steady state in which aerobic ATP production keeps pace with demand
little lactic acid accumulates under steady state conditions
depletion of glycogen and blood glucose, together with the loss of fluid and electrolytes through sweating, set limits on endurance and performance even when lactic acid does not

8. Fatigue
muscle fatigue - progressive weakness and loss of contractility from prolonged use of the muscles
repeated squeezing of rubber ball
holding text book out level to the floor
causes of muscle fatigue
ATP synthesis declines as glycogen is consumed
ATP shortage slows down the Na+ - K+ pumps
compromises their ability to maintain the resting membrane potential and excitability of the muscle fibers
lactic acid lowers pH of sarcoplasm
inhibits enzymes involved in contraction, ATP synthesis, and other aspects of muscle function
release of K+ with each action potential causes the accumulation of extracellular K+
hyperpolarizes the cell and makes the muscle fiber less excitable
motor nerve fibers use up their ACh
less capable of stimulating muscle fibers – junctional fatigue
central nervous system, where all motor commands originate, fatigues by unknown processes, so there is less signal output to the skeletal muscles

9. Endurance
endurance – the ability to maintain high-intensity exercise for more than 4 to 5 minutes
determined in large part by one’s maximum oxygen uptake (VO2max)
maximum oxygen uptake – the point at which the rate of oxygen consumption reaches a plateau and does not increase further with an added workload
proportional to body size
peaks at around age 20
usually greater in males than females
can be twice as great in trained endurance athletes as in untrained person
results in twice the ATP production

10. Oxygen Debt heavy breathing continues after strenuous exercise
excess post-exercise oxygen consumption (EPOC) – the difference between the resting rate of oxygen consumption and the elevated rate following exercise.
typically about 11 liters extra is needed after strenuous exercise
repaying the oxygen debt
needed for the following purposes:
replace oxygen reserves depleted in the first minute of exercise
oxygen bound to myoglobin and blood hemoglobin, oxygen dissolved in blood plasma and other extracellular fluid, and oxygen in the air in the lungs
replenishing the phosphagen system
synthesizing ATP and using some of it to donate the phosphate groups back to creatine until resting levels of ATP and CP are restored
oxidizing lactic acid
80% of lactic acid produced by muscles enter bloodstream
reconverted to pyruvic acid in the kidneys, cardiac muscle, and especially the liver
liver converts most of the pyruvic acid back to glucose to replenish the glycogen stores of the muscle.
serving the elevated metabolic rate
occurs while the body temperature remains elevated by exercise and consumes more oxygen

11. Beating Muscle Fatigue (Do NOT Try this at Home!)
Taking oral creatine increases level of creatine phosphate in muscle tissue and increases speed of ATP regeneration
useful in burst type exercises – weight-lifting
risks are not well known
muscle cramping, electrolyte imbalances, dehydration, water retention, stroke
kidney disease from overloading kidney with metabolite creatinine
carbohydrate loading – dietary regimen
packs extra glycogen into muscle cells
extra glycogen is hydrophilic and adds 2.7 g water/ g glycogen
athletes feel sense of heaviness outweighs benefits of extra available glycogen

12. Physiological Classes of Muscle Fibers
slow oxidative (SO), slow-twitch, red, or type I fibers
abundant mitochondria, myoglobin and capillaries - deep red color
adapted for aerobic respiration and fatigue resistance
relative long twitch lasting about 100 msec
soleus of calf and postural muscles of the back
fast glycolytic (FG), fast-twitch, white, or type II fibers
fibers are well adapted for quick responses, but not for fatigue resistance
rich in enzymes of phosphagen and glycogen-lactic acid systems generate lactic acid causing fatigue
poor in mitochondria, myoglobin, and blood capillaries which gives pale appearance
SR releases & reabsorbs Ca+2 quickly so contractions are quicker
(7.5 msec/twitch)
extrinsic eye muscles, gastrocnemius and biceps brachii
ratio of different fiber types have genetic predisposition – born sprinter
muscles differ in fiber types - gastrocnemius is predominantly FG for quick movements (jumping)
soleus is predominantly SO used for endurance (jogging)

13. FG and SO Muscle Fibers Figure 11.20 FG SO
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Figure 11.20